[LB7 (OP)]

In the device:


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I built a device, to measure the distance. I fixed a meter on the red wall. I fixed a meter on the circle. I can move only the circle in one axis in translation and I can rotate it around itself. I can only rotate the red wall. With another people I measured several times the distance like that: one people rotate the red wall slowly, the other people moves the circle in translation and rotates it to follow the millimeters (one millimeter after one) on the rules, I think like the millimeters are tooth of gears. I will add photos after. And what ? the distance is well d2 not d1.

[LB7 (OP)]

it works !!! just need to measure it in reality, you can do the experience ! I'm right !!!

Experience it is what you like no ? It is easy, you need only 30 minutes.

Take a pan lid (inox) for the circle and take a wood rod for the red wall, the friction is enough to have no slip: no energy must be created by friction, I use that experience to measure the distance of rotation of the circle and compare it to the length of the wood between the 2 dots of contact. Move the circle in horizontal translation and rotate the wood rod, that's all: the distance of rotation is EQUAL to d2.

I will post a video.